The standard Weberian definition of a government is an institution that has a monopoly (or near monopoly) on the legitimate use of violence within a certain geographical area. The way it achieves this monopoly is by keeping out other competing sovereigns (aka nations) that would be that monopoly of legitimate coercion. The two main ways our government does that is by keeping the militaries of other nations out of the United States and by stopping insurgents or potential insurgents from seizing power through violence and supplanting the U.S. government.
U.S. immigration laws are not primarily designed or intended to keep out foreign armies, spies, or insurgents. The main effect of our immigration laws is to keep out willing foreign workers from selling their labor to willing American purchasers. Such economic controls do not aid in the maintenance of national sovereignty and relaxing or removing them would not infringe upon the government’s national sovereignty any more than a policy of unilateral free trade would. If the United States would return to its 1790-1875 immigration policy, foreign militaries crossing U.S. borders would be countered by the U.S. military. Allowing the free flow of non-violent and healthy foreign nationals does nothing to diminish the U.S. government’s legitimate monopoly of force.
As endless examples from history show, there are other ways to obtain control over a territory besides direct military action. A tangible, recent example comes from Crimea. Russia was able to seize control of Crimea not primarily through force of arms but through force of demographics. Most residents of Crimea are ethnic Russians and they voted overwhelmingly (even discounting for fraud) to be a part of Russia. If most residents of Crimea were ethnic Ukrainians, would they vote to join Russia? Not likely.
In the U.S., libertarians  and gays  have tried efforts with the same goal: obtaining power not through force but through demographics. While colonialism usually consisted of a small group of foreigners controlling a large group of native inhabitants, in many cases a conquest was followed by resettling the territory with residents of the conquering power. Stalin moving Russians into the Baltic states helped the USSR maintain control over those states. Look at a map of Europe over time for many other examples. Even the Geneva Convention acknowledges such attempts: "The Occupying Power shall not deport or transfer parts of its own civilian population into the territory it occupies." 
Now, with all that history in your mind, ask yourself what China would do if we opened our borders as Alex Nowrasteh wants. The leaders of China have thousands of years of history behind them and they take the long view. They're also much more patriotic to China than any U.S. libertarians are to the U.S. Would they just think that the U.S. having open borders was an interesting news story and move on, or would they seek to take advantage of the situation?
If they were smart - and they are - they'd look into how they could use the situation to help China. Wouldn't China having increased influence on U.S. policies - such as our relationship with Japan and Taiwan - benefit China? How could China use our open borders policy to increase their influence on U.S. policies? Simply by sending us people. China wouldn't need to send their Army, they'd just send us millions of people. Some of those people would end up becoming loyal to the U.S., but most would continue to be loyal to China. They'd represent a political power bloc inside the U.S. for China. Some U.S. politicians would have to worry how the millions of Chinese living inside the U.S. would react to U.S. policies, but other U.S. politicians would partly or mostly represent the interests of that population. Leftwing NGOs would form alliances with groups linked to the Chinese government and partly represent Chinese interests. Can anyone not see Leland Yee and groups like the American Civil Liberties Union in such roles? Under Alex Nowrasteh's policies, China could expand into the U.S. and obtain political power inside the U.S. just as it's doing in Africa.
It's unrealistic to expect that those millions of foreigners could stay here for years or decades as Gastarbeiter, unable to become citizens. The ACLU, other far-left NGOs, and perhaps the Democratic Party would agitate to make them citizens just as they're doing with current illegal aliens.
Other BRIC countries wouldn't be far behind China: Brazil and India could send us people in order to obtain a political power bloc inside the U.S. to push their agendas. Russia could send people to Alaska; while the recent secession petition  might have been a joke mostly signed by foreigners, but if a majority of residents of Alaska were ethnic Russians who knows what might happen? While Russia is in much worse shape financially than the U.S., ethnicity and nationalism tend to be stronger than economic considerations for most people.
The Mexican government has political power inside the U.S. because so many Mexicans live here: see that link for dozens of examples. That government has direct or indirect links to pro-illegal immigration groups (American Civil Liberties Union, National Council of La Raza, NALEO, Illinois Coalition for Immigrant and Refugee Rights, etc. etc.) and politicians (Jared Polis, Bob Menendez, etc. etc.) Those groups and politicians all push that country's agenda inside the U.S. Politicians like Gil Cedillo represent both Americans and foreign citizens, and likewise for many members of the California legislature. An Illinois politician, Martin Sandoval, served as a state senator as he also served on an advisory board to the Mexican government. By sending us people, Mexico has been able to obtain power inside the U.S. Whether they did that intentionally or not, it happened.
Libertarians live in a simplistic fantasy world where millions - perhaps hundreds of millions - of people would come here just to work and make money. The world isn't that simple: leaders of populous or expansionist countries wouldn't abide by Nowrasteh's childish view of the world. They'd take what they could get for their own people, Americans be damned.
Nowrasteh continues on:
Many of those who complain that free immigration would reduce U.S. national sovereignty really mean that the U.S. government will have less power. That is absolutely correct. If free immigration was the law of the land then the government would not be able to arbitrarily stop immigrants for any virtually any reason, the power of American bureaucrats to capriciously exclude immigrants and punish American businesses who want to hire them would be diminished, the outcomes of attempting to immigrate would be ex ante more predictable for the immigrant, and the U.S. government’s power in relation to immigration would be brought in line with our common law traditions...
Here's the problem: the U.S. government isn't some foreign entity, the U.S. government is (at least ideally) us: We the People. The U.S. government is supposed to represent the collective interests of Americans. We American citizens have decided collectively that an influx of hundreds of millions of foreign workers would be bad for our economy. Some like Nowrasteh would say that's wrong, but that's what we've decided. Don't worry, Alex Nowrasteh has a workaround: dilute the power of current American citizens to decide our fate. Nowrasteh would take some power away from most Americans and put it in the hands of business owners and foreign citizens. They could influence immigration matters and U.S. citizens would have a reduced role.
In other words, Nowrasteh's own policies would reduce the sovereignty of current U.S. citizens, reducing our ability to decide our own policies.
Want to do something about this? Nowrasteh is a True Believer so nothing you can say would sway him. However, what you can do is make the points above to anyone who enables him. Search for @AlexNowrasteh and if the person tweeting him seems non-delusional make the points above to them. When you see him quoted in a news article, look up the reporter and make the points above to them.
 cato . org/blog/national-sovereignty-free-immigration-are-compatible
 freestateproject . org/about/101-reasons-move-new-hampshire
 dailykos . com/story/2011/10/02/1021933/
 icrc . org/ihl.nsf/WebART/380-600056
 "Thousands sign WhiteHouse.gov petition for Alaska to secede -- to Russia"
foxnews . com/politics/2014/03/27/
Tue, 04/29/2014 - 13:43 · Importance: 4